Many modern metrology systems rely on optical analysis of interactions between illumination emanating from at least one illumination source and the surface of a sample being analyzed. Characteristics of an illumination beam provided by an illumination source may change over time as a result of deterioration or movement of illumination source components. Accordingly, the illumination source may need to be recalibrated from time to time to compensate for changes of illumination beam characteristics, such as minimum beam spot size and/or location, beam caustic, and the like.
Current methods for measuring illumination beam characteristics are time consuming and often impractical for on-site calibration of an illumination source. For example, one current method of measuring illumination beam characteristics entails illumination scattering from particles deposited on a wafer to analyze defects with properly aligned scattering collection optics. As such, it would be advantageous to provide a method for illumination beam characterization that cure the defects of the prior.